Final Objective: This module is designed to give you a safe and practical approach to paediatric anaesthesia.

Enabling Objective: To achieve this goal, you should be able to:

Outline the anatomical and physiological differences between adults and children and describe how these affect anaesthetic practice (including the response to drugs).

Know the important aspects in the conduct of infant and child anaesthesia.

Understand the equipment used in paediatric anaesthesia

Reference Reading:

Developing Anaesthesia Chapters 25, 26, 27 & 28.

Oxford handbook of anaesthesia chapter 33.

Children, especially neonates and children weighing less than about 15 kg, differ markedly from adults. There are differences in size, anatomy, physiology, response to drugs and psychology. The anaesthetist must assess the child preoperatively, gain the trust of both the child and parents and prepare meticulously for anaesthesia.

Paediatric anatomy, physiology and pharmacology.

Babies have a relatively larger head with a prominent occiput. The head needs to be stabilised for intubation but may not require a pillow below the head. The neck is short and the tongue large. If excessive submandibular pressure is applied during mask ventilation, the tongue may obstruct the airway. Infants and neonates breathe mainly though their noses which are small and easily obstructed. Nasal resistance accounts for almost 50% of total airway resistance.

The larynx is more anterior and is situated at a higher level relative to the cervical vertebrae (C3 to C4 at birth) compared to an adult (C6). The epiglottis is relatively longer, leaf like and U shaped. The inexperienced anaesthetist may find the baby more difficult to intubate.

The trachea is short. The distance from the glottis to the carina is about 4 cm in a full term neonate. Tube fixation is essential. Extension of the head can result in extubation and flexion can result in endobronchial intubation. Endotracheal tubes must be securely taped and the number marking on the tube closest to the lip noted. With most infants, if the 10 cm mark on the endotracheal tube is at the gums, the tip of the tube will be just above the carina. In older children the length of the endotracheal tube may be estimated by (age/2) + 12 cm.

The narrowest part of the upper airway is the cricoid ring in the pre-pubertal child. After puberty, the narrowest part of the airway is at the level of the vocal cords. The diameter of the trachea in the newborn is 4 to 5 mm. Just 1 mm of oedema can cause serious harm. Children before puberty should have an uncuffed tube and there should be a slight air leak with positive pressure ventilation. It is important to select the correct size endotracheal tube. If resistance occurs during intubation, the next smaller tube should be tried.

Paediatric endotracheal size and age

Premature

2.5 - 3.0 mm

Neonate – 6 months

3.0 - 3.5 mm

6 months – 1 year

3.5 mm – 4.0 mm

Greater than 1 year

(Age/4) + 4

Their ribs are more horizontal and any increase in the volume of the thorax is due to downward movement of the diaphragm. A distended abdomen or surgical retraction can easily reduce ventilation.

Oxygen consumption in neonates may be greater than 6 ml/kg/min (twice the oxygen consumption of adults). In infancy a gradual change towards the adult rate (3.5 ml/kg) occurs. A higher oxygen consumption means that neonates and infants will rapidly consume their oxygen reserves and become cyanotic if they are apnoeic. Attempts at intubation must not exceed 30 seconds. Higher oxygen consumption leads to a higher carbon dioxide production, which requires increased ventilation to remove it. The increased ventilation is mainly achieved by a higher respiratory rate (newborn 35 to 40 breaths/minute). The tidal volume/kg is similar for adults and children.

Premature and ex-premature babies up to 52 weeks post conceptual age are at risk of apnoea after general anaesthesia. They must be very closely observed for at least 24 hours.

Cardiac output in the neonate maybe 200 to 400 ml/kg/min compared to 70 to 80 ml/kg/min in the adult because of the higher metabolic rate and oxygen requirement in the neonate. Stroke volume is relatively fixed in the newborn due to the poorly compliant ventricular muscle. Stroke volume in the newborn is 5 to 7 ml/kg compared to 1 to 2 ml/kg in adults. Therefore, an increase in cardiac output is achieved by an increase in heart rate. The newborn’s resting heart rate is much higher than that of the adult (130 to 140/min in the neonate, 70/min in the adult) and it is not until about the age of ten that it reaches adult rates. Bradycardia occurs in response to hypoxia.

The sympathetic nervous system is not well developed. Infants can easily become bradycardic. Atropine premedication will reduce the incidence of bradycardia and reduce secretions. (Intravenous or intramuscular dose is 0.01 to 0.02 mg/kg). The maximum dose should be less than 0.06 mg/kg.

Blood pressure is lower in children than adults because of low peripheral resistance. Blood volume in the neonate is about 80 ml/kg compared to 70 ml/kg in the adult. Haemoglobin at birth is high (18 g/dl) and falls to a low at 3 to 6 months of about 11 g/dl. The change is due to a decrease in foetal haemoglobin. A haemoglobin of less than 13 g/dl in the newborn and less than 10 g/dl in the first 6 months of life may be significant.

Paediatric Cardiovascular Parameters.

Age Weight (kg)

Heart Rate Blood Pressure

Newborn

3.5

120

80/40

3 months

6.0

140

95/55

6 months

7.5

140

95/55

1 year

10

125

95/65

3 year

14

100

100/60

7 year

22

90

100/70

10 year

30

80

105/70

14 year

50

80

120/70

Neonates have greater total body water (70 to 75% of body weight) compared to adults (60% of body weight). There is a larger extracellular compartment (ECF) and smaller intracellular compartment (ICF). By the first year of age the proportions are the same as for adults (ECF 45%, ICF 55% of total body water). The increased metabolic rate of infants results in a faster turnover of extracellular fluid. An interruption of the normal fluid intake can therefore rapidly lead to dehydration and the anaesthetist must take care with fluid management. The anaesthetist must estimate replacement fluid, maintenance fluid and ongoing fluid losses.

The maintenance fluid requirements of an infant is 4 ml/kg/h for the first 10 kg, adding 2 ml/kg/h for the second 10kg and 1 ml/kg/h for each kg over 20 kg. [For example a 16 kg child needs (10 kg x 4 ml) + (6 kg x 2 ml) = 52 ml/h or 1248 ml/day] Well children should be given 0.45% NaCl with 5% glucose (+/- 20 mmol KCl/l). Unwell children need 0.9% NaCl.

Maintenance fluid volume will need to be reduced (70% maintenance) in many unwell children: i.e. children with suspected neurological [meningitis and encephalitis] and respiratory [bronchiolitis and pneumonia] disease. Postoperative use of 4% dextrose and 1/5 N/Saline is no longer recommended due to the risk of hyponatraemia.

Children have a relatively small blood volume and should be calculated prior to surgery. (Neonate 90 ml/kg, infant 85 ml/kg and child 80 ml/kg). A 5kg infant will have a blood volume of only 400 ml. Blood loss of only 40 ml is a 10% decrease in blood volume and 80 ml a 20% loss of blood volume. A soaked swab will contain at least 5 ml and a small pack at least 20 ml of blood.

The neonate has decreased glomerular filtration and tubular function. Renal clearance of drugs and their metabolites is reduced during the first year. The ability to excrete a fluid load is initially poor but this function rapidly increases in the first month of life. The ability to produce concentrated urine is also initially poor and improves rapidly in the first two months reaching adult levels by two years of age. Urine output should be at least 0.5 ml/kg/h.

The newborn is at a greater risk of cooling when exposed to a cold environment because the ratio of body surface area to body weight is double that of older patients. Skin and subcutaneous fat is thinner, providing less insulation and leading to greater heat loss. Heat production is low and the ability to shiver is not well developed. Temperature regulation is immature. The environmental temperature range in which oxygen consumption is minimal (thermoneutral range) is narrow. A decrease in environmental temperature of 2 degrees Celsius may double the oxygen consumption of a newborn. Infants must be kept warm. The operating theatre should be heated and the infant kept covered. Try to warm intravenous fluids.

Liver metabolism may be poor in the newborn but develops rapidly in the first few weeks. Drugs such as opioids, benzodiazepines and barbiturates may not be metabolised as rapidly in neonates.

The differences in physiology of the infant will alter the effect of some drugs. All opioids and central nervous system depressants must be given with caution in neonates unless the patient is being ventilated and closely monitored. Morphine clearance in neonates is one quarter that of adults so that the elimination half time will be four times that of adults. The immature respiratory centre makes the neonate more sensitive to the respiratory depressive effects of morphine.

The proportion of cardiac output going to the brain is greater in the neonate than in older children. The dose of intravenous induction agents should be reduced in neonates.

Decreased renal and liver function results in certain drugs being excreted more slowly. The dosing interval should be increased to avoid toxicity.

Neonates and infants require a greater dose suxamethonium (2 mg/kg) than adults (1 mg/kg). The MAC of inhalational agents is greater in the young (peaking at 1 year of age approximately 50% greater than adult values) and decreases with increasing age, however neonates require lower concentrations than infants do. There may be nearly a 30% greater anaesthetic requirement for inhalation agents but there is a smaller margin of safety between adequate anaesthesia and cardiovascular and respiratory depression in infants compared with adults. Both induction and recovery from inhalation agents is more rapid in children than adults.

Anaesthesia for infants and children.

During the preoperative assessment of children it is very important to assess the severity of any upper respiratory tract infection, which is very common in preschool children. Elective surgery should be cancelled if the child is unwell with a high fever and has signs and symptoms of a respiratory tract infection. These children are at risk of laryngospasm, bronchospasm and hypoxia and surgery should be delayed 6 weeks. If the upper respiratory tract infection is mild, then the anaesthetist should decide if the surgery should be delayed (2 weeks). The anaesthetist must consider the size and type of surgery, the planned anaesthetic technique and the size and medical history of the child. The smaller the child and pre-existing respiratory disease significantly increases the risk. Previously unreported murmurs are commonly heard at 2-4 years of age. The majority are functional. A pansystolic murmur with normal heart sounds in a child with a normal oxygen saturation, no limitation in exercise tolerance and no failure to thrive can be assumed to be innocent. If there are any doubts, surgery should be delayed until a complete cardiac assessment has been made.

The child must be weighed. Drugs need to be given accurately based on weight. As with adults, the child needs assessment of their heart and lungs. The airway should be assessed. Children often have loose teeth. The anaesthetist should ask about previous anaesthetics and about a family history of anaesthetic problems.

Premedication may be useful to produce preoperative sedation but is not routine. Injections should be avoided. Sedative drugs include benzodiazepines (e.g. diazepam, midazolam), chloral hydrate and antihistamines (e.g. trimeprazine, promethazine). Choral hydrate (40 mg/kg) has been used safely and effectively for many years but has a bitter taste. Midazolam may be given orally (0.5 mg/kg up to a maximum of 20 mg) or intranasal (0.3 mg/kg) and produces sedation within 30 minutes. Midazolam can rarely cause respiratory depression. Occasionally it may make the child hyperactive. For oral administration it should be mixed with a small amount of clear sweet liquid (e.g. apple juice). Ketamine may also be used as premedication orally (5 mg/kg).

Paracetamol is very effective in providing analgesia and can be given orally or rectally. It should be given at least 30 minutes before the operation. The initial maximum dose is 30 mg/kg. Further doses should be 15 mg/kg every 4 hours. The rectal dose is 30 mg/kg initially then 20 mg/kg every 6 hours. The maximum daily dose during the first two days is 90 mg/kg. After two days the maximum daily dose should not exceed 60 mg/kg.

Opioids are best avoided as premedication unless the child is in severe pain. Premedication with intramuscular anticholinergics is not recommended. If vagolytic drugs are indicated, they are usually administered intravenously at the time of induction.

Small children and babies are more likely to become distressed with fasting. Clear fluids, up to two hours preoperatively, reduce anxiety and may decrease gastric volume. Neonates and infants may have breast milk up to four hours preoperatively. Fasting time for cow’s milk, solids and formula should be six hours. It is important not to fast children and babies for too long, as they have a smaller glycogen store which puts them at risk of hypoglycaemia. Premature infants cannot maintain adequate blood sugar for any period of fasting. Severe hypoglycaemia can result in apnoea, convulsions and brain damage. Premature infants should have intravenous dextrose whilst they are fasting.

Before induction of anaesthesia, prepare all drugs and equipment. Check the child’s weight. Pre calculate the dose of atropine, suxamethonium and adrenaline.

Both intravenous and inhalation induction have their strengths and weaknesses. Children with a full stomach need a rapid sequence induction/intubation. Traditional inhalation induction is commenced with 100% oxygen, then nitrous oxide is introduced and then the inhalation agent is gradually increased. Older children often tolerate a facemask, but smaller children may better accept a cupped hand. Allowing the small child to sit up, leaning against the anaesthetists chest, during inhalation induction may be less frightening than forcing the child to lie down on the operating table. If during induction, vital signs become abnormal, the concentration of inhalation agent is reduced or even discontinued. Often there is some degree of airway obstruction as the depth of anaesthesia increases. Obtaining a tight mask fit and creating a small amount of positive end expiratory pressure can correct this.

Intravenous induction may be achieved with thiopentone (3 – 5 mg/kg), propofol (1 – 3 mg/kg) or ketamine (1 – 2 mg/kg). Ketamine may also be given intramuscularly (5 – 10mg/kg). The use of a topical local anaesthetic agent (e.g EMLA cream) over the vein to be cannulated makes venous cannulation less traumatic.

Laryngospasm is more common in children than adults. Children become cyanosed more rapidly than adults because of increased metabolic rate/oxygen consumption and reduced FRC. Bradycardia is a premorbid event indicating inadequate cardiac output. Partial laryngospasm is managed by providing 100% oxygen, continuous positive airway pressure (CPAP), gentle assisted ventilation and propofol 1-2 mg/kg bolus. Complete laryngospasm or deteriorating laryngospasm requires early administration of suxamethonium 1-2 mg/kg. Atropine 10 mg/kg may also be necessary.

All neonates and children feel pain. The principles of postoperative pain relief are the same as adults including multimodal analgesia and specialised pain charts. Assessment with infants and neonates can be difficult.

Anaesthetic equipment.

Paediatric airway equipment must have a small dead space and minimal resistance to breathing. Transparent masks allows for observation of cyanosis/regurgitation and the presence of breathing. Rendall-Baker/Soucek masks, that were developed from molds of the facial contours of children, have less dead space than the clear mask with an inflatable rim. It is essential that the mask chosen is the appropriate size. The most common error in mask ventilation is to cause obstruction of the airway by compressing the pharyngeal space by placing the fingers below the mandibular ridge and squeezing too tightly.

Oropharyngeal airways are available in a range of sizes from 000 to 4 but are rarely useful in neonates who are obligate nasal breathers but may be advantageous in older children. The correct length is equal to the distance from incisors to the angle of the jaw. If the airway extends too deeply, its tip may push the epiglottis down, causing traumatic epiglottitis and airway obstruction. If the airway is too short, it rests against the base of the tongue, forcing it posteriorly and causing airway obstruction.

Nasopharyngeal airways have a limited use in paediatric anaesthesia. The correct length is equal to the distance from the tip of the nose to the tragus of the ear.

The indications and insertion of laryngeal mask airways are similar to adult use. They are available in several sizes, however their use in small infants can easily result in airway complications.

No. 1: 0-5 kg

No. 1.5: 5-10 kg

No. 2: 10-20 kg

No. 2.5: 20-30 kg

No. 3: > 30 kg

Laryngoscope blades are available in different lengths from size 0-3 and may be curved or straight. Generally, a straight blade design (e.g Miller) is better in infants and neonates. During laryngoscopy with a straight blade the tip of the blade is used to elevate the epiglottis.

Uncuffed endotracheal tubes, which allow an air leak at 20 cm H2O, are generally used in paediatric anaesthesia. Unlike adults, the narrowest part of the paediatric trachea is at the level of the cricoid ring. If the tracheal tube is too large, it will compress the tracheal epithelium at this level, leading to ischaemia and possibly subglottic stenosis. Uncuffed tubes are available from 2 to 7 mm internal diameter (I.D). Cuffed tubes start from 5.5 mm I.D. A full term neonate usually requires a tube of 3 mm I.D, a 1 year-old 3.5 to 4.0 mm I.D and a 2 year-old 4.5 to 5.0 mm ID. Over 2 years of age, tube size may be estimated by 4 + age (years)/4. Endotracheal tubes at least one size smaller and larger must also be available.

There are a number of different breathing systems suitable for use in paediatric anaesthesia. The Mapleson E (Ayres’ T piece) is the most commonly used circuit in paediatric anaesthesia. It is valveless, low resistance, simple and lightweight and has a small dead space. The volume of the expiratory limb should be greater than the patient’s tidal volume. It is very suitable for children less than 20 kg. Fresh gas flows of 2 to 3 times minute volume should be used to prevent re-breathing during spontaneous ventilation or a minimum of 6 litres/min. For controlled ventilation a flow rate of 1000 ml + 100 ml/kg should prevent re-breathing.

The open ended 500 ml reservoir bad (or Jackson-Rees modification) allows assessment of tidal volume, application of CPAP and the capability for assisted or controlled ventilation. A hand should be on the reservoir bag at all times during spontaneous ventilation to help monitor the effectiveness of ventilation and to provide CPAP if needed to maintain airway patency.

For children above 20 kg, adult breathing systems are suitable for both spontaneous and controlled ventilation. A circle breathing system can be used safely for controlled ventilation in children heavier than 10 kg if the deadspace is reduced by using smaller tubing, Y piece and connectors.

Many adult ventilators cannot be safely used for paediatric patients. They cannot reliably deliver the small tidal volumes, and rapid respiratory rates required. Pressure controlled ventilation is commonly used and reduces the risk of barotrauma. Mot children can be ventilated adequately with inspiratory pressures of 16–20 cmH20 and a respiratory rate between 16-24 breaths/minute.

Standard monitoring includes close, continuous observation by the anaesthetist. A precordial or oesophagealstethoscope can be used to assess breath sounds, heart rate, rhythm and the intensity of heart sounds. The precordial stethoscope should be firmly taped on the chest wall over the apex of the heart.

Accurate blood pressure measurement requires the correct size cuff. The cuff should cover at least two thirds of the upper arm and the inflatable bag should almost encircle the arm. If the cuff is too small, a reading that is falsely high may be obtained. If the cuff is too large the reading may be falsely low.

Temperature monitoring is very important in children. General anaesthesia decreases the temperature threshold at which the body initiates thermoregulation in response to cold stress. Mild intraoperative hypothermia is common and results from a combination of (1) Approximately 20% reduction in metabolic heat production, (2) increased environmental exposure, (3) anaesthesia induced central thermoregulation inhibition and (4) redistribution of heat within the body.

Paediatric patients differ from adults in having a larger surface area/body weight ratio leading to increased evaporative heat losses. The patient should be kept covered as much as possible and operating theatres should be warm.

Pulse oximetry is particularly important because of the rapid rate of desaturation in infants and children.

A volume-limiting device should be placed between the intravenous fluid container and the patient to prevent accidental fluid overload. Generally an intravenous set with a microdrop outlet (60 drops/ml) is used for smaller children. Urine output should be at least 0.5 ml/kg/h.

Pain management.

Some principals of paediatric pain management are similar to pain management in adults. Pain management should follow the WHO analgesic ladder. Combined analgesia is more effective than single modality analgesia.

Pain must be regularly and appropriately assessed. Anaesthetists must believe the patient’s or family's reports of pain and choose pain control options appropriate for the patient, family and setting. Pain treatment must be given in a timely, logical and coordinated fashion and patients must be reviewed within in a short time.

The assessment of pain in children is more difficult. Although self-reporting of pain is the gold standard for assessment of the site, nature and severity of pain, it is not always applicable in children. Assessment must be appropriate to the child’s developmental stage using their language (ouch! hurt, sore). Non-verbal children are very vulnerable to having their pain underestimated. Their assessment requires the use of physiological and behavioural markers. Although physiological and behavioural responses are very sensitive indicators of pain, they have poor specificity; and can occur with apprehension, stress related disease, and discomfort.

What are the advantages and disadvantages of inhalation and intravenous induction in children?

How do you manage laryngospasm?

What are the correct dosages of common anaesthetic and emergency drugs?

How may hypothermia be prevented in children?

What are the correct fasting times for infants and children?

ASSIGNMENT

Discuss the major differences in anaesthesia between adults and children. What are the greatest risks and difficulties in providing paediatric anaesthesia?

PAEDIATRIC ANAESTHESIA CASE STUDIES

Case No 16.1

The nurse calls you to the post anaesthetic care unit because after an uneventful tonsillectomy, a 2-year boy is tachypnoeic (40 breaths/minute) with stridor and intercostal recession.

What are the likely causes?

What would you do?

Case No 16.2

A 12-year old boy is undergoing a wound debridement and manipulation of a compound fractured femur. During the surgery his heart rate increases to 140 beats/minute and his capillary refill is greater than 3 seconds.

He weighs 35 kg. Estimate his blood volume.

What are the potential causes of his tachycardia?

How do you differentiate between these causes?

Outline your management.

Case No 16.3

An eight-year old girl has a left sided posterior mandibular swelling which is painful. She refuses to talk and is holding a cloth up to her mouth.

Discuss your preoperative assessment.

What are your major anaesthetic concerns?

Outline your anaesthetic plan.

Case No 16.4

A six-year old girl has had left sided lower abdominal pain and fever for 4 days. She has been eating and drinking very little.

How can you assess her degree of dehydration?

Discuss what intravenous fluid she needs before and after surgery.

How would you administer her anaesthetic?

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